1. Field of the Invention
The present invention relates to droplet deposition apparatus, such as, for example, ink jet printheads.
2. Description of the Related Art
The current drive in drop-on-demand inkjet printing towards higher resolution requires increased density both of ink ejection nozzles and the associated drive circuitry. However, the increased density of the drive circuitry can lead to problems associated with overheating. Similarly, the trend towards ever greater printhead widths places correspondingly greater demands on heat management within printheads. Thermal (xe2x80x9cbubble jetxe2x80x9d) printheads benefit in this regard from having their drive circuitry in close contact with the ink, which has a cooling effect. This is offset, however, by the need for special measures to maintain the electrical integrity of the circuitry in the ink environment.
It is an object of at least the preferred embodiments of the present invention to prevent, in a simple manner, the drive circuitry of a printhead from overheating without at the same time risking its electrical integrity.
In a first aspect the present invention provides droplet deposition apparatus comprising:
a fluid chamber having actuator means actuable by electrical signals to effect ejection of droplets from the fluid chamber;
drive circuit means for supplying the electrical signals to the actuator means; and
conduit means for conveying droplet fluid to or from said fluid chamber;
the drive circuit means being in substantial thermal contact with said conduit means so as to transfer a substantial part of the heat generated in said drive circuit to the droplet fluid.
Arranging the drive circuit means in such a manner can conveniently allow the ink in the printhead to serve as the sink for the heat generated in the drive circuitry. This can substantially reduce the likelihood of overheating, whilst avoiding the problems with electrical integrity that might occur were the integrated circuit packaging containing the circuitry allowed to come into direct contact with the ink.
The apparatus may comprise first conduit means for supplying droplet fluid to said fluid chamber and second conduit means for leading droplet fluid from said fluid chamber. If so, the drive circuit means may advantageously be thermally connected to the second conduit means. This can provide the most direct route out of the printhead for the heat generated in the chip of the drive circuit and, in the event that the heat produced by the chip varies significantly during operation, can minimise any variation in the temperature of the ink in the fluid chamber itself. As is known, for example, from WO97/35167, such temperature variation can give rise to variations in droplet ejection velocity and consequent dot placement errors in the printed image.
Where the drive circuit is incorporated within an integrated circuit package of substantially cuboid form in which at least some of the faces are rectangles each having a surface area, a face other than that face having the smallest surface area may advantageously be arranged so as to lie substantially parallel to the direction of fluid flow in that part of the conduit closest to said face, and to be in substantial thermal contact with the fluid. Such an arrangement can ensure significant heat transfer to the droplet fluid. Preferably, that face having the greatest surface area is arranged so as to lie parallel to the direction of fluid flow. Circuit architecture permitting, such an arrangement can maximise heat transfer from the circuitry.
A second aspect of the present invention provides droplet deposition apparatus comprising:
at least one droplet election unit comprising a plurality of fluid chambers, actuator means and a plurality of nozzles arranged in a row, said actuator means being actuable to eject a droplet of fluid from a fluid chamber through a respective nozzle; and
a support member for said at least one droplet ejection unit, said support member comprising at least one droplet fluid passageway communicating with said plurality of fluid chambers and arranged so as to convey droplet fluid to or from said fluid chambers in a direction substantially parallel to said nozzle row and to transfer a substantial part of the heat generated during droplet ejection to said conveyed droplet fluid.
This can provide for substantially even distribution of heat along the length of the support member, which can lead to reduced thermally-induced strains that might otherwise distort the printhead. Such distortion would become more pronounced as the width of the printhead increased, for example, to that of a page (typically 12.6 inches. (32 cm) for the American xe2x80x9cFoolscapxe2x80x9d standard) and would occur regardless of whether a plurality of narrow ejection units or a single wide ejection unit were used in conjunction with the support member.
Advantageously, the droplet fluid passageway may occupy the majority of the area of the support member when viewed in cross-section. Alternatively or in addition, the passageway may comprise respective portions for the flow of droplet fluid in to and out of each fluid chamber. Such flow can aid the transfer of heat from the fluid chamber (where the main source of heatxe2x80x94the actuator meansxe2x80x94is located) to the remainder of the support, thereby reducing temperature differentials.
To provide effective support for the at least one droplet ejection unit, the cross-section of support member is preferably wider in the direction of ink ejection from the nozzles than in the direction of the nozzle row.
In one embodiment, the apparatus comprises a plurality of said droplet ejection units, the support member supporting the droplet ejection units side by side in the direction of the nozzle rows, the support member comprising at least one droplet fluid passageway communicating with at least two of said ejection units and arranged so as to convey droplet fluid to or from said ejection units in a direction substantially parallel to said nozzle rows and to transfer a substantial part of the heat generated during droplet ejection to said conveyed droplet fluid.
Heat distribution may be facilitated by constructing the support member from a materialxe2x80x94such as aluminiumxe2x80x94having a high thermal conductivity. Such a material also has advantages as regards manufacture and cost. Problems arise, however, where the ejection unit is made from material having a coefficient of thermal expansion that is significantly different to that of the support. This will be the case with an ejection unit comprising channels formed in a body of piezoelectric material (typically lead zirconium titanate, PZT) described hereafter. As will be readily appreciated, differential expansionxe2x80x94particularly in the direction of the nozzle row in a xe2x80x9cpagewidexe2x80x9d devicexe2x80x94may lead to distortion and/or breakage of ink seals, actuator components, electrical contacts, etc.
Therefore, it is preferable to provide means for attaching said at least one droplet ejection unit to the support member in order to substantially avoid transferral of thermal deformation of the support member to said at least one droplet ejection unit.
A third aspect of the present invention provides droplet deposition apparatus comprising:
a fluid chamber, at least part of which is formed from a first material having a first coefficient of thermal expansion, said chamber being associated with actuator means actuable to eject a droplet from the chamber and having a port for the inlet of droplet fluid thereto;
a support member for said fluid chamber and including a passageway for supply of droplet liquid to said port, the support member being defined at least in part by a second material having a second coefficient of thermal expansion greater than said first coefficient; and
means for attaching the fluid chamber to the support member in order to substantially avoid transfer of thermal deformation of the support member to said fluid chamber.
Preferably, the attachment means comprises resilient bonding means for bonding the or each fluid chamber to the support member. In an example described hereafter, an adhesive rubber pad is used to bond a support member of extruded aluminium to a fluid chamber structure comprising a channel formed in a body of PZT and closed by cover member of a material, such as molybdenum, that is thermally matched to the PZT. Forming ink supply ports in the cover and ink ejection nozzles in the channelled component can provide a particularly compact design having a low component count.
Further advantageous embodiments of the invention are set out in the description, drawings and dependent claims.